1. Field of the Invention
The present invention relates to the structure of a semiconductor power device and a control device thereof. The semiconductor power device is selectable for thyristor operation or transistor operation as an IGBT (Insulated Gate Bipolar Transistor), specifically having two gate electrodes, and the control device makes the semiconductor power device having two gate electrodes operable by a single signal.
2. Description of the Prior Art
Recently, the quality of power semiconductor devices has been rapidly enhanced. With regard to, for instance, bipolar transistors, they have been designed so as to have high quality and to withstand high voltages and to be able to operate at a high current handling capability. Moreover, there have been proposed intelligent modules having various built-in protection functions. In addition, there have been proposed conductivity-modulation transistors (i) capable of responding at a high speed. These power semiconductor devices have been adopted in small-sized electric appliances which are miniaturized so as to be driven by a battery and to be easily carried about or used in electric motorcars, adoption of which has been investigated from the viewpoint of environmental protection, and a partial contribution to electric power-saving. In order to cope with the recent increasingly high demand for electric power, there has been a requirement for the development of power semiconductor devices which allow a further saving of electric power and a reduction of power loss.
For instance, there have been developed MCT's (MOS controlled thyristor) for reducing the ON-state voltage through the use of a thyristor structure, for imparting fast operability to devices and for reducing the electric power required for driving these devices. Such an MCT is a device having a structure and an equivalent circuit as shown in FIGS. 1 and 2, respectively, and is detailed in the article of V.A.K. Temple entitled "MOS controlled thyristors" (IEEE International Electron Device Meeting Digest, 1984). MCT 160 comprises an ON-FET 162a and an OFF-FET 162b controlled by a single gate electrode 161 and when an electric current is passed through ON-FET 162a and OFF-FET 162b is simultaneously shut-off, an npnp thyristor is in the turn-on state, which comprises an n.sup.+ -cathode layer 164 provided thereon with a cathode electrode 163, a p.sup.- -base layer 165, an n-base layer 166 and a p.sup.+ -anode layer 167 provided thereon with an anode electrode 168. On the other hand, when an electric current is passed through OFF-FET 162b and ON-FET 162a is simultaneously shut-off, n- base layer 166 and anode electrode 168 are short-circuited, a pnp transistor 169 is in the OFF state and hence this device is in the turn-OFF state. This pnp transistor 169 comprises p- -base layer 165, n-base layer 166 and p.sup.+ -anode layer 167. Since MCT 160 puts the device in the ON state through the action of a MOS in such a manner, the response speed thereof is very high. Further, it also puts the thyristor in the ON state and, therefore, the ON state voltage drop of the device is very low, on the order of about 1 V. However, as shown in FIG. 3, the carrier densities of holes and electrons present within anode layer 167 and base layers 165 and 166 are high in the thyristor state. For this reason, the turn-off time of this device is quite long, on the order of 2 to 3.mu. sec due to such very high carrier densities, and thus the loss during this time becomes a problem. In particular, this results in a substantial increase in electric power loss when the device is used in high frequency applications.
In an IGBT (Insulated Gate Bipolar Transistor) as shown in FIG. 4, on the other hand, the collector current during the turn-off state has a typical wave form as shown in FIG. 5. As seen from FIG. 5, the wave form during the turnoff state comprises a first phase 191 and a second phase 192. First phase 191 corresponds to a phenomenon where a channel formed by the action of a gate 187 disappears, the supply of electron current from an emitter electrode 188 to an n- -base layer 182 is interrupted and correspondingly, the current is rapidly reduced instantaneously in proportion thereto. Second phase 192 corresponds to a phenomenon where the carriers remaining in n.sup.- -base layer 182 migrate through the action of a pnp transistor comprising a p.sup.+ -collector layer 181, n.sup.- -base layer 182 and a p-base layer 183 and are reduced due to disappearance of the carriers through recombination within the life time .tau. thereof. Reduction of the turn-off time of the IGBT, therefore, can be accomplished by reducing the injection level of hole current, or by shortening the carrier life time .tau.. For this reason, there has been proposed a technique in which an n.sup.+ -layer is formed between p.sup.+ -collector layer 181 and n.sup.- -base layer 182 to control the injection level of the hole current (see IEEE, IEDM Technical Digest, 4.3 (1983), pp.79-82) and a technique in which the impurity doping concentration of collector layer 181 is controlled. In addition, as a method for shortening the carrier life time .tau., there have been proposed techniques which make use of a life time control process such as irradiation with an electron beam or heavy metal diffusion (see IEEE, Trans. Electron Devices, ED-31 (1984), pp. 1790-1795). These techniques make it possible to reduce the fall time during the turn-off state to approximately 200 nsec and to thus obtain a device capable of being operated even at a high frequency. Incidentally, techniques concerning this IGBT are detailed in an article by the inventors of this invention ("New IGBT Modules with Improved Power Loss at High Frequency PWM Model", Electronics, '90 Munchen).
As has been discussed above in detail, an IGBT has an advantage in that it has a short turn-off time, but is a device which has a high ON-state voltage drop in the order of about 2 V and in which it is difficult to reduce the ON-state loss. This is because the density of p-base layer 183 cannot be sufficiently increased to prevent the occurrence of a latch-up state due to operation of a parasitic thyristor. In an IGBT, the emitter current I.sub.E is equal to I.sub.h +I.sub.MOS as shown in FIG. 4 and if the gain of the pnp transistor comprising p-base region 183, an n.sup.- -drift region 182 and p.sup.+ -collector region 181 is assumed to be (.alpha..sub.PNP, the following relation can be obtained: EQU I.sub.h =(.alpha..sub.PNP /(1-.alpha..sub.PNP)).times.I.sub.MOS
Therefore, the following equation can be obtained: EQU I.sub.E (1/(1-.alpha..sub.PNP)).times.I.sub.MOS
I.sub.h (hole current) and hence the current of the IGBT vary depending on the value of .alpha..sub.PNP. In the foregoing relations, Imos means an electron current.
One of the most important key technologies for solving the problem of improvement of quality, miniaturization and cost-saving in the field of power electronics, is to reduce the power loss of power devices. This requires the development of a power device having a short turn-off time and simultaneously having a low ON-state voltage drop. However, if the same means used in an IGBT, i.e., the reduction of carrier life time and formation of an n.sup.+ -buffer layer, are adopted to reduce the turn-off time in, for instance, the MCT discussed above, the ON state voltage drop increases as in the case of an IGBT. For this reason, it is not possible to make use of the advantage of an MCT, i.e., a low ON-state voltage drop. Moreover, it is necessary to instantaneously extract the carriers accumulated in an MCT in order to reduce the turn-off time. However this requires the formation of an additional MOS gate for extracting the current, and the driving power must be increased to extract the carriers. Further, an instantaneous extraction of a large current requires the use of a MOS gate having a low ON-state resistance. It is presently possible to make the most use of the properties of each device such as low ON-state voltage drop or short turn-off time separately, but the development of a device having these properties in combination is very difficult.
Accordingly, in light of the foregoing problems, an object of the present invention is to provide a new power device which makes the most use of the characteristic peculiar to an MCT, i.e., a low ON-state voltage drop, and the characteristic peculiar to an IGBT, i.e, a short turn-off time.
A problem with the double gate semiconductor device when used in inverters or the like is that the control terminals have to be individually driven. That is, whereas a prior art power device has only a single control terminal connecting the gate electrode, and the power device can be controlled only by supplying a signal to drive the control terminal, in the double gate semiconductor device, two signals meeting the two control terminals connecting to the two gate electrodes have to be prepared. Furthermore, unless the two control signals are appropriately controlled, since the device does not positively transfer from the thyristor condition to the transistor condition, nor can it be turned off, for inverters and the like, this tends to cause generation of an arm short-circuit.
Furthermore, since the double gate semiconductor device cannot be turned off unless it is transferred from the thyristor condition to the transistor condition, even when trouble occurs at the initial state where the device becomes conductive, it is necessary to first transfer the device to the thyristor condition, and there is the possibility of the occurrence of unrestorable damage during the transition. Furthermore, since, even if trouble occurs during operation, immediate shut-down is difficult, it is necessary to find the trouble during operation as early as possible and take measures to transfer to the transistor condition.
Therefore, it is another object of the present invention to provide a control device applied to a double gate semiconductor device having advantageous characteristics such as high speed and reduced power, which is possible to be handled as conventional power devices and protect the double gate semiconductor device from troubles such as an abnormal current and the like.